1. Field of the Invention
The present invention relates to an injection and compression molding apparatus for plastics and a molding method therefor. In one aspect the invention relates to an injection and compression molding apparatus for plastics and a molding method therefor in which the feed of molten resin into molds, a pressure apparatus and a molding method therefor are improved.
2. Description of Related Art
In conventional injection molding operations, since molten resin is supplied into a closed mold cavity through a gate, orientation occurs in the resin, and strong residual strain remains in a portion adjacent to the gate owing to the high pressure at the gate. Curvature and twisting therefore occur in molded products, and physical properties in a portion near the gate deteriorate. A method of resolving this problem is an injection and compression (or press) molding method in which molten resin is supplied by injection into molds which are appropriately opened and which are then closed to effect compression molding. In this method, however, if the molten resin flows backward through the gate during clamping compression, sinking (sink mark) occurs in a portion of the molded article near the gate, and the weight of the resin in the molded article is reduced, resulting in the deterioration in the stability of the properties of the resulting molded products. In order to resolve this problem during clamping compression, several methods have been proposed, e.g., a first method of effecting compression after the gate has been closed (after the resin at the gate has solidified), a second method of closing a supply port (Japanese Patent Laid-Open No. 61-22917), a third method of controlling compression pressure (Japanese Patent Laid-Open No. 61-83016) and so on.
In injection and compression molding by the first method, however, compression must be effected after the gate has been closed (the resin at the gate is solidified), and thus compression cannot be properly timed easily. In addition, if the gate size is reduced in order to speed up the sealing of the gate, injection of the resin requires more time, and products having appropriate sizes are not obtained in some cases. In the case of the second method in which the supply port is closed, if compression is performed early, the resin flows backwardly, and the closing of the supply port and compression cannot be easily timed because the supply port cannot be easily, completely and rapidly closed. In the third method of controlling compression pressure, since there are many conditional limitations for preventing any counter flow, it is difficult to set conditions. The second method and third method also involve the problem that the presence of a sprue runner makes the methods difficult.
In the second method, mold clamping is stopped or the speed of molding clamping is reduced during the operation of supplying molten resin into the cavity, Mold clamping is again started or the speed of mold clamping is increased at the same time as or immediately before the molten resin is completely supplied, and press molding is then performed. However, the second method involves the problem that if a movable platen is precisely stopped or the speed of movement thereof is reduced during the course of movement, the movable platen is moved backwardly or inclined by the reaction force of the molten resin supplied into the cavity of the molds, resulting in a change in a compression margin and the formation of defective molded products. The second method also involves the following problems. Since the movement of the movable platen is unstable, i.e., the movable platen can be moved backwardly, or compression is not effected in a parallel state, molded products cannot be stably obtained. Since molten resin is supplied to the molds which are appropriately opened, and the molds are then clamped for compression molding, if the closing of the supply port and compression are not well timed when the supply port is closed, the molten resin can flow backwardly through the gate during clamping compression. Hence, sinking occurs in a portion of the molded article near the gate or the weight of the resin in the molded article is reduced, and thus molded products cannot be stably obtained.
In consideration of the aforementioned problems, it is an object of the present invention to provide an injection and compression molding apparatus and a molding method therefor which are capable of stably producing molded products having good appearance and dimensional precision and high quality with good productivity and having no sink mark in the portion near the resin supply port.